1. Field of the Invention
The present invention relates to a semiconductor device substrate (a substrate for a semiconductor device) which is composed of monocrystal formed on an insulator and is useful widely for large-scale integrated circuits and the like. The present invention also relates to a process for producing the above semiconductor device substrate.
2. Related Background Art
A process for producing a semiconductor device substrate through formation of monocrystalline Si semiconductor layer on an insulator is well known as silicon-on-insulator technique (SOI). This SOI technique is widely utilized in device formation, since the SOI technique has many advantages which are not achievable with a bulk Si substrate production. The advantages brought about by the SOI technique are as below:
1. Ease of dielectric separation, and practicability of high integration,
2. High resistance against radioactive rays,
3. Low floating capacity, and practicability of high speed operation,
4. Practicability of omission of a welling step,
5. Practicability of prevention of latching-up,
6. Practicability of thin film formation for complete depletion type field effect transistor, and so forth.
The SOI technique is described in literature, for example: Special Issue: "Single-crystal silicon on non-single-crystal insulators" edited by G. W. Cullen: Journal of Crystal Growth, Vol. 63, No. 3, pp. 429-590 (1983).
Prior to the SOI technique, an SOS technique (silicon-on-sapphire) is known which forms heteroepitaxial Si on monocrystalline sapphire substrate by CVD (chemical vapor deposition). This SOS technique is not widely applied because of many crystal defects caused by insufficient coherency of the lattice at the interface between the Si layer and the underlying sapphire substrate, migration of aluminum from the sapphire substrate to the Si layer, and, above all, the high cost of the substrate and difficulty in enlarging the size thereof.
In recent years, the SOI structure without use of the sapphire substrate is going to be realized. This attempt is made in two ways: a first method comprises steps of oxidizing a surface of an monocrystalline Si substrate, forming an aperture in the oxidized layer to uncover partially the Si substrate, growing Si epitaxially in a lateral direction using the uncovered Si as the seed to form an monocrystalline Si layer on the SiO.sub.2 (Si layer being deposited on SiO.sub.2); and a second method comprises steps of forming SiO.sub.2 under the monocrystalline Si substrate by use of the monocrystalline Si substrate itself as the active layer (no Si layer being deposited).
Of the above first method, the step of a solid-phase epitaxial growth is described below in detail. The solid-phase epitaxial growth is classified into vertical solid-phase epitaxial growth and lateral solid-phase epitaxial growth. The vertical solid-phase epitaxial growth is employed chiefly for crystallinity recovery after ion implantation. For formation of the SOI structure, the lateral solid-phase epitaxial growth is suitable. In the formation of the SOI structure by the lateral solid-phase epitaxial growth, SiO.sub.2 having an aperture is formed on an Si substrate; amorphous Si is deposited over the entire face of the substrate; the vertical solid-phase epitaxial growth is carried out through the aperture; and then the lateral solid-phase epitaxial growth is carried out over the formed SiO.sub.2. The solid-phase epitaxial growth is practicable at temperature of as low as about 600.degree. C.
The SOI structure formation through the above-described solid-phase epitaxial growth involves problems below:
In the lateral solid-phase epitaxial growth, all the monocrystalline Si region has to be formed in the face direction, which necessitates growth in long distance and long time. The long growth time causes formation of nuclei in the amorphous Si, which inhibits the growth, and results in insufficient growth distance of only several .mu.m. To solve this problem, countermeasures are taken such as adjustment of the pressure and beam irradiation to extend the growth distance, and introduction of an impurity to increase the growth velocity. However, with any of the aforementioned countermeasures, the growth distance in lateral direction is limited to several tens of .mu.m at the largest, and an SOI of a larger area cannot be formed. Furthermore, the lateral solid-phase epitaxial growth is known to result in remarkably low crystallinity than the vertical solid-phase epitaxial growth and to cause a number of dislocation and twinning, which impairs directly the device properties.
Known techniques for forming the SOI structure by deposition include direct lateral epitaxial growth of monocrystalline Si by CVD; deposition of amorphous Si and subsequent lateral solid-phase epitaxial growth by heat treatment; irradiation of a focused energy beam such as electron beam and laser light beam on polycrystalline Si layer to grow monocrystalline layer on SiO.sub.2 by melt-recrystallization; and scanning with a bar-shaped heater over a molten zone (zone melting recrystallization).
These methods, however, involve many problems in controllability, productivity, uniformity, quality, etc., and are not promising in industrial application. For example, the CVD process requires sacrificial oxidation for forming a thin flat film. The lateral solid-phase growth results in low crystallinity. The beam annealing process involves problems in treating time by a converged beam, control of superposition of the beam and focus adjustment. Of the above methods, zone melting recrystallization is most highly developed, and has been employed for experimental production of relatively large integrated circuits. This method, however, still causes crystal defects in subgrain boundary, etc., and does not give a minority carrier device.
Furthermore, conventional solid-phase epitaxial growth methods essentially require uncovered monocrystalline Si as the seed on the substrate surface, and cannot be practiced by use of a substrate other than monocrystalline Si.
Atsutoshi Doi, et al. reported that SOI was prepared by attaching and pressing a patterned monocrystalline silicon external seed on an amorphous silicon layer and then utilizing the crystalline region grown in solid phase by heat treatment as a seed for laser recrystallization in place of using a substrated as the seed for the laser recrystallization (A. Doi, et al., Appl. phys. Lett., 59, 2518 (1991); Tuda, et al., 1991 Autumn Applied Physics Society Lecture Preliminary Report, p. 621, 9p-C-2; Doi, et al., 1991 Spring Applied Physics Society Lecture Preliminary Report, p. 614, 28p-X-11; Doi, et al., 1990 Spring Applied Physics Society Lecture Preliminary Report, p. 608, 29p-ZF-7). This method is a laser recrystallization method having some problems in treating time of scanning a converged beam and control of superposition of the beam and focus adjustment and poor productivity, and therefore it is not industrially applied. Furthermore, since the seed for the laser recrystallization is formed by a solid-phase epitaxial growth from the external seed, the steps thereof are complicated and finally the crystallinity of the epitaxial Si layer remarkably goes bad in comparison with that of the original monocrystalline external seed. In addition, there is a problem that the external seed can not easily removed since it is bonded to the epitaxial layer by valence bond.
Koichiro Hoh, et al. reported that monocrystalline Si was brought in contact with the surface of amorphous Si deposited on SiO.sub.2 and crystallization was carried out from it as a seed (Uzawa, et al., 41th Semiconductor Integrated Circuit Technology Symposium Lecture Papers, p. 37 (1991); K. Hoh, et al., Denki Kagaku, 59, 1079 (1991); Uzawa, et al., 1991 Autumn Applied Physics Society Lecture Preliminary Report, p. 621, 9p-C-1; Yasuda, et al., 1991 Spring Applied Physics Society Lecture Preliminary Report, p. 614, 28p-X-10). In this method, it is basically impossible to remove an external seed. Therefore, they has attempted that amorphous Si is epitaxially grown from metallic silicide as a seed which is obtained by contacting monocrystalline Si deposited with metal and epitaxially growing metallic silicide.
In case of Ni silicide, the lattice coherency with si is very good, but the crystallization temperature of Ni silicide is 750.degree. C. or higher. Therefore, there is a problem that generation of nuclei in amorphous Si occurs to form polycrystal before epitaxial growth of Ni silicide. Furthermore, there is a problem that the lattice coherency of Co silicide having low crystallization temperature with Si is not good.
On basis of available data of X-ray diffraction, although the crystallinity of epitaxial Si is not evaluated in detail, a good crystallinity is not expected because of heteroepitaxial growth and solid-phase growth.
Accordingly, this method can not satisfy formation of a good epitaxial Si layer and removal of seed at the same time.
A substrate made of a different material such as light-transmissive substrate typified by glass, allows a deposited thin Si layer to grow only into an amorphous or polycrystalline layer under the influence of the disorderness of the crystal structure of the substrate, and therefore is unsuitable for production of devices of high performance. It is due to the amorphousness or the difference in periodicity of crystal structure of the substrate. Therefore, simple deposition of Si layer will not give excellent monocrystal layer.
The light-transmissive substrate is important in constructing a light-receiving device for a contact sensor, a projection type liquid crystal image displaying apparatus, and the like. In order to provide a sensor or image element (picture element) of a display apparatus in higher density, higher resolution, and higher fineness, high performance of the driving device is required. Therefore, the device on a light-transmissive substrate have to be made from monocrystal layer having excellent crystallinity.
Thus, amorphous Si or polycrystalline Si will not generally give a driving device which exhibits the satisfactory performance required nowadays or to be required in the future because of many defects in the crystal structure.
The formation of monocrystalline Si on another kind of material is an important technique in forming a three-dimensional combination with another functional device or in use, as a substrate, of material having properties not obtained by Si such as light transmissivity, high heat releasability, high mechanical strength, and low cost. However, the formation of excellent monocrystalline Si film on such a material is extremely difficult.
Recently in addition to the above conventional SOI formation processes, another method of forming SOI structure is attracting attention in which a monocrystalline Si substrate is bonded to another thermally oxidized monocrystalline Si substrate by heat treatment or with an adhesive. This method requires an active layer formed in a uniform thin film for the device. That is, the monocrystalline Si substrate of several hundred microns in thickness has to be made into a thin film of several micron or thinner.
This thin film formation is practicable in two ways: (1) thin film formation by polishing, and (2) thin film formation by selective etching.
In the former method, i.e., polishing, it is not easy to form a uniform thin film. In particular, in formation of a film of submicron thick, the variation of the film thickness amounts to several tens of %, which is a serious problem. Moreover, for larger diameter of wafer, the thin film formation is much more difficult.
The latter method, i.e., the selective etching, although it is considered to be effective in formation of a uniform thin film, involves problems such that the selectivity is 10.sup.2 or lower and is insufficient; the surface property is inferior after etching; the crystallinity of the SOI layer is low as the result of utilizing ion implantation and epitaxial growth or heteroepitaxial growth on an Si layer doped with B in high density (C. Harendt, et al., J. Elect. Mater., Vol. 20, p. 267 (1991); H. Baumgart, et al., Extended Abstract of ECS 1st International Symposium of Water Bonding, p. 733 (1991); and C. E. Hunt, Extended Abstract of ECS 1st International Symposium of water Bonding, p. 696 (1991)).
Accordingly, the SOI preparation by bonding at present has many problems in the controllability and the uniformity. Furthermore, preparation of light-transmissive SOI by bonding has a serious problem of difference of thermal expansion coefficients.